Fediverse is, well, federated. This means that it's spread across multiple servers that are independently run by their admins, but have agreed to work together and share communities between each other.
For example, lemmy.world is a site operating several communities, as are beehaw.org and lemmy.ml. One of them could choose to monetize, and that wouldn't affect the other sites. If new site pops up that is full of ads and spam, then other sites could decide to block communities from them on their site.